Diffuser for centrifugal compressor and method of producing the same

ABSTRACT

At least the throat of the suction surface of a conventionally-used diffuser blade having a two-dimensional shape is cut into a concave shape with a ball end mill. In the throat, the nearer to the bottom of the concave shape, the higher the gas pressure becomes, and therefore the gas flows from the concave bottom toward the hub end and the shroud end thereby to reduce the thickness of the boundary layer formed on the hub end and the shroud end.

TECHNICAL FIELD

The present invention relates to a diffuser for a centrifugal compressorand a fabricating method and, in particular, to a diffuser for acentrifugal compressor and a fabricating method capable of reducing boththe incidence loss.

BACKGROUND ART

A centrifugal compressor is well known as a type of compressor forcompressing and increasing the pressure of a gas.

FIG. 1 is a sectional view of a centrifugal compressor comprising animpeller 12 mounted on a rotary shaft 11 and a casing 13. The casing 13has formed therein an impeller accommodating portion, a diffuser portionand a spiral scroll.

A blade 14 is mounted on the impeller 12, and the gas flowing in alongthe direction of arrow 15 is accelerated by the impeller 12 and flowsout along the radial direction 16 of the centrifugal compressor. Thediffuser portion on the outer periphery of the impeller 12 includes adiffuser blade 17 for converting the dynamic pressure of the fluidflowing out along the radial direction 16 into a static pressure.Specifically, the diffuser blade 17 is mounted on an annular disk 18fitted in the front inner wall of the diffuser portion of the casing 13,and the diffuser blade 17 extends toward the rear inner wall (along theheight) of the diffuser portion in the casing.

FIG. 2 is a perspective view of the conventional diffuser comprising aplurality of two-dimensional diffuser blade 17 mounted on the annulardisk 18. The cross section A-A of the diffuser blade 17 is rectangular.The connection between each side wall of the diffuser blade 17 and theannular disk 18 is defined by an arcuate surface, and the curvesconnecting the center of the radius of curvature of the arcuate surfacesextend smoothly along the diffuser blade 17 and intersect to each other.At the upstream side of the diffuser blade 17, a ridge line is formed bythe intersection of the arcuate surfaces to prevent stalling andsurging. This conventional diffuser blade is proposed and disclosed in,for example, Japanese Unexamined Patent Publication No. 10-77997.

The gas flowing out of the impeller 12 flows into the diffuser portionin the direction of arrow 20. The angle between the velocity vector ofthe gas and an axis tangent to the circumference of the annular disk 18is referred to as flow angle α. The distribution of the flow angle αalong the height of the diffuser blade has a parabolic shape wherein theangle is substantially zero at one side where the diffuser blade 17 ismounted to the annular disk 18 (shroud end S) and at the other side rearthe inner wall of the casing (hub end H), while the flow angle α ispositive in the intermediate area therebetween.

FIG. 3 is a graph showing the distribution of the flow angle along theheight of the diffuser blade, in which the abscissa is the height of thediffuser blade and the ordinate is the flow angle α. Each diffuser blade17 is mounted obliquely on the annular disk 18 in a direction (lengthdirection) from the impeller of the diffuser mounted portion toward thescroll. The surface impinged by the gas flowing into the diffuser 17 iscalled a pressure surface, and the surface opposite to the pressuresurface a suction surface. The angles of the suction surface and thepressure surface relative to the axis tangent to the circumference ofthe annular disk 18 are referred to as a suction surface blade angleα_(ksuc) and a pressure surface blade angle α_(kpre), respectively. Thesuction surface blade angle α_(ksuc) and the pressure surface bladeangle α_(kpre) are substantially constant over the height of thediffuser blade 17.

The difference (α−α_(ksuc)) between the flow angle α and the suctionsurface blade angle α_(ksuc) is defined as an incidence In. Theincidence In becomes negative near the hub end H and the shroud end S(hatched portions in FIG. 3) of the diffuser blade 17. As a result, theboundary layer thickness increases, which results in increase in a losson the suction surface near the hub end H and the shroud end S of thediffuser blade 17.

DISCLOSURE OF THE INVENTION

The present invention has been achieved in view of the problemsdescribed above, and the object thereof is to provide a diffuser for thecentrifugal compressor and a fabrication method capable of reducing thepressure loss by suppressing the generation of the boundary layer at thesuction surface near the hub end and the shroud end of each diffuserblade.

According to a first aspect of the invention, there is provided adiffuser for a centrifugal compressor comprising a diffuser bladeincluding a suction surface which is concave toward a pressure surfacein a cross section, perpendicular to the flow direction.

According to a second aspect of the invention, there is provided adiffuser for a centrifugal compressor comprising a diffuser bladeincluding a suction surface which defines an acute angle, in a crosssection perpendicular to the flow direction, in relation to at least oneof a shroud surface and a hub surface.

According to a third aspect of the invention, there is provided adiffuser for a centrifugal compressor comprising a diffuser bladeincluding a suction surface which is concave toward a pressure surfacein a cross section, perpendicular to the flow direction at least in athroat portion and a portion upstream thereof.

According to a fourth aspect of the invention, there is provided adiffuser for a centrifugal compressor comprising a suction surface whichdefines an acute angle, in a cross section perpendicular to the flowdirection at least in a throat portion and a portion upstream thereof,in relation to at least one of a shroud surface and a hub surface.

According the first to fourth aspects of the invention, the suctionsurface is formed into a concave surface at least in the throat portionto suppress the generation of a boundary layer near the shroud surfaceand the hub surface.

According to a fifth aspect of the invention, there is provided a methodof fabricating a diffuser for a centrifugal compressor, comprising thesteps of fabricating a two-dimensional blade; and cutting one of theside surfaces of the blade, fabricated in the preceding step, into aconcave shape with a ball end mill.

In the fifth aspect of the invention, the diffuser blade is fabricatedby cutting one side surface of the diffuser blade having atwo-dimensional shape with a ball end mill.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view of a centrifugal compressor.

FIG. 2 is a perspective view of a diffuser conventionally used.

FIG. 3 is a graph showing the distribution of the flow angle along theheight of the diffuser blade.

FIG. 4A is a perspective view of a diffuser blade according to theinvention.

FIG. 4B is a sectional view of a diffuser blade taken in line I in FIG.4A.

FIG. 4C is a sectional view of a diffuser blade taken in line II in FIG.4A.

FIG. 4D is a sectional view of a diffuser blade taken in line III inFIG. 4A.

FIG. 4E is a sectional view of a diffuser blade taken in line IV in FIG.4A.

FIG. 4F is a sectional view of a diffuser blade taken in line V in FIG.4A.

FIG. 5 is a graph showing the distribution of the flow angle along theheight of a diffuser blade according to the invention.

FIG. 6 is a sectional view of a second diffuser blade used with adiffuser according to the invention.

FIG. 7A is a top plan view of a diffuser according to the invention.

FIG. 7B is a sectional view of a diffuser blade taken in line B-B inFIG. 7A.

FIGS. 8A to 8C are diagrams showing the steps of fabricating a diffuserblade used with a diffuser according to the invention.

FIG. 8D is a sectional view showing the portion indicated by III in FIG.8C.

FIG. 8E is a sectional view showing the portions indicated by II and IVin FIG. 8C.

FIG. 8F is a sectional view showing the portions indicated by I and V inFIG. 8C.

BEST MODE FOR CARRYING OUT THE INVENTION

FIG. 4A is a perspective view of a diffuser blade used in a diffuseraccording to the invention, and FIGS. 4B to 4F are sectional views fromthe upstream side taken in lines I to V, respectively. A suction surface401 of a diffuser blade 4 is formed into a concave surface toward anopposite pressure surface 402. Specifically, at least one of angle δHwhich is defined by the suction surface and inner wall of the casingnear a hub end H and angle δS which is defined by the suction surfaceand, an annular disk near a shroud end S of the diffuser blade 4 isacute.

In FIGS. 4A to 4F, the suction surface is formed as a concave surfacetoward the pressure surface.

FIG. 5 is a graph showing the distribution of the flow angle along theheight of the diffuser blade. The suction surface angle α_(ksuc)decreases near the hub end H and the shroud end S of the diffuser blade4 and, on the other hand, increases at the central portion along theheight of the diffuser blade 4. Therefore, the incidence In=(α_(ksuc)−α)of the diffuser blade according to the invention assumes a negativevalue near the hub end H and the shroud end S of the diffuser blade 4,while the absolute value of the incidence In is smaller than theconventional diffuser blade.

This indicates that the boundary layer thickness of the suction surfacenear the hub end H and the shroud end S of the diffuser blade 4 isreduced thereby leading to a smaller incidence loss than theconventional diffuser blade.

FIG. 6 is a sectional view of a second diffuser blade according to theinvention, which is configured of not a curved surface but of bent flatsurfaces.

FIG. 7A is a top plan view of the diffuser according to the invention,and FIG. 7B a sectional view of the diffuser blade along line B-B inFIG. 7A. The constant-pressure curves of the gas between the twodiffuser blades 41, 42 are shown in FIG. 7A. Area between the normalline C-C to the center line along the length of the first diffuser blade41 from the upstream end of the second diffuser blade 42 and theupstream end of the first diffuser blade 41 is defined as a throat, andthe length from the upstream end of the first diffuser blade 41 to thenormal line C-C is defined as a throat length XT. In the throat areafrom the upstream end of the first diffuser blade 41 to the throat lineC-C, the pressure distribution is in the shape of a fan centered at theupstream end of the second diffuser blade 42. The pressure in the area“a” connecting the upstream ends of the blade is low, while the pressurein the area “b” connecting the upstream end of the second diffuser blade42 and the center of the throat area of the first diffuser blade 42 ishigh.

FIG. 7B is a sectional view along line B-B perpendicular to the centerline along the length of the first diffuser blade 41 in the throat area.The gas pressure distribution is such that the higher the pressure, thenearer to the first diffuser blade 41. Thus, the gas develops asecondary flow 43 from the suction surface of the first diffuser blade41 toward the pressure surface of the second diffuser blade 42. As aresult, the boundary layer thickness on the suction surface of the firstdiffuser blade 41 near the hub end H and the shroud end S is reducedwhile. Therefore the incidence loss is reduced.

In the area downstream of the throat area where the first diffuser blade41 and the second diffuser blade 42 overlap each other, theconstant-pressure curves assume a form perpendicular to the center linealong the length of the first diffuser blade 41 and the second diffuserblade 42. Therefore, the gas is prevented from flowing from the suctionsurface of the first diffuser blade 41 toward the pressure surface ofthe second diffuser blade 42. Thus, it is especially important to form athroat and a concave suction surface at upstream of the throat, whilethe portion downstream of the throat is not required to be formed into aconcave shape.

FIGS. 8A to 8F are diagrams for explaining the steps of fabricating adiffuser blade according to this invention. First, a two-dimensionaldiffuser blade having a rectangular cross section is fabricated (FIG.8A).

Next, the suction surface of the two-dimensional diffuser blade is cutwith the ball end mill thereby to form a concave suction surface. Aftercutting the two-dimensional diffuser blade along one end thereof (FIG.8B), a concave suction surface is formed by cutting along the other end(FIG. 8C). The angles δH and δS that the suction surface forms with thehub surface and the shroud surface, respectively, of the diffuser bladeare acute.

The cutting depth of the ball end mill being constant, the steps offabricating the diffuser blade according to this invention aresimplified.

Normally, a two-dimensional diffuser blade has the greatest thickness atthe longitudinally central portion thereof, and therefor the crosssection of the diffuser blade according to the invention is symmetriclongitudinally about the central portion (III) thereof. Specifically,the cross section of the central portion (III) is shaped as shown inFIG. 8D. The cross sections of the portions (II) and (IV) are shaped asshown in FIG. 8E, and the cross sections of the portions (I) and (V) asshown in FIG. 8F.

The embodiment of the invention described above refers to an applicationof the diffuser according to the invention to a centrifugal compressor.It is apparent, however, that the diffuser according to this inventionis also applicable to a centrifugal blower and a centrifugal pump withequal effect.

With the diffuser for a centrifugal compressor according to thisinvention, the boundary layer generated on the suction surface near theshroud end and the hub end of the diffuser blade is reduced, whereby theincidence loss is reduced.

In the method of fabricating the diffuser for the centrifugal compressoraccording to this invention, the diffuser blade can be easily fabricatedby cutting a two-dimensional diffuser blade with a ball end mill.

1. A diffuser for a centrifugal compressor comprising a diffuser bladeincluding a suction surface which is concave toward a pressure surfacein a cross section, perpendicular to the flow direction.
 2. A diffuserfor a centrifugal compressor comprising a diffuser blade including aconcave suction surface which defines an acute angle, in a cross sectionperpendicular to the flow direction, in relation to at least one of ashroud surface and a hub surface.
 3. A diffuser for a centrifugalcompressor comprising a diffuser blade including a suction surface whichis concave toward a pressure surface in a cross section, perpendicularto the flow direction at least in a throat portion or a portion upstreamthereof.
 4. A diffuser for a centrifugal compressor comprising a concavesuction surface which defines an acute angle, in a cross sectionperpendicular to the flow direction at least in a throat portion or aportion upstream thereof, in relation to at least one of a shroudsurface and a hub surface.
 5. A method of fabricating a diffuser for acentrifugal compressor comprising the steps of fabricating atwo-dimensional blade, and cutting one of the side surfaces of theblade, fabricated in the preceding step, into a concave shape with aball-end mill.